docs: def warn(self, message, container=None):
          """Present the warning `message` to the user, adding information on the
          location of the related element in the input file."""
          if self.source is not None:
              message = '[{}] '.format(self.source.location) + message
          if container is not None:
              try:
                  message += ' (page {})'.format(container.page.formatted_number)
              except AttributeError:
                  pass
          warn(message)